RelayCommand - MVVMLight 4.5 vs previous

Before updating to MVVMLight 4.5 we would construct RelayCommands in our viewmodels with lambdas. Both execute and canexcute callbacks are fired when bound to a button. Specifially, this is in an .NET 4.5 WPF application.

new RelayCommand(() => Execute(), CanExecute)

However after updating to MVVMLight 4.5, in the example above CanExecute is invoked but
() => Execute() does not get invoked.